I am not sure I get the girl's attitude towards Bud. I get she felt insecure about living where she did, so why try to date Bud if she felt that way?

It seemed strangely important to her to go the that astronomy thing. So important that when Kathy mentioned the birthday Friday night (it was Margaret's birthday Friday night, not just Friday), the girl wouldn't mention it to Bud.

Like they couldn't go to a movie the next night and go to the observatory some other time?